What is callback in Android? [duplicate]

I want to understand the concept of callback. I have searched on internet about the callbacks and there are many examples using interface, and one class is calling a method of another class using that interface. But still I can't get the main concept of callbacks, what is the purpose of using callbacks?

Solution 1:

Here is a nice tutorial, which describes callbacks and the use-case well.

The concept of callbacks is to inform a class synchronous / asynchronous if some work in another class is done. Some call it the Hollywood principle: "Don't call us we call you".

Here's a example:

class A implements ICallback {
     MyObject o;
     B b = new B(this, someParameter);

     @Override
     public void callback(MyObject o){
           this.o = o;
     }
}

class B {
     ICallback ic;
     B(ICallback ic, someParameter){
         this.ic = ic;
     }

    new Thread(new Runnable(){
         public void run(){
             // some calculation
             ic.callback(myObject)
         }
    }).start(); 
}

interface ICallback{
    public void callback(MyObject o);
}

Class A calls Class B to get some work done in a Thread. If the Thread finished the work, it will inform Class A over the callback and provide the results. So there is no need for polling or something. You will get the results as soon as they are available.

In Android Callbacks are used f.e. between Activities and Fragments. Because Fragments should be modular you can define a callback in the Fragment to call methods in the Activity.

Solution 2:

You create an interface first, then define a method, which would act as a callback. In this example we would have two classes, one classA and another classB

Interface:

public interface OnCustomEventListener{
  public void onEvent();   //method, which can have parameters
}

the listener itself in classB (we only set the listener in classB)

private OnCustomEventListener mListener; //listener field

//setting the listener
public void setCustomEventListener(OnCustomEventListener eventListener) {
   this.mListener=eventListener;
}

in classA, how we start listening for whatever classB has to tell

classB.setCustomEventListener(new OnCustomEventListener(){
    public void onEvent(){
       //do whatever you want to do when the event is performed.
    }
});  

how do we trigger an event from classB (for example on button pressed)

if(this.mListener!=null){
   this.mListener.onEvent();
}

P.S. Your custom listener may have as many parameters as you want

Source